A linearly polarized laser beam is emitted from a laser oscillator in a laser beam machine. When a workpiece (an object to be machined) is machined with using this laser beam, a light absorption of the workpiece largely differs depending on a movement direction of the workpiece, which disables stable laser machining to be performed. Thus, the linearly polarized laser beam emitted from the laser oscillator is converted to circularly polarized light with using a waveplate.
FIG. 4 is a view for describing an optical system of a conventional laser beam machine. The optical system of the laser beam machine is composed of a transmission optical system and a processing optical system. The transmission optical system is an optical system that guides laser light to the processing optical system and, is mainly composed of a circularly polarizing mirror 20, and a plurality of zero shift mirrors (including a metallic reflecting mirror) 21. On the other hand, the processing optical system is an optical system that condenses the laser light on a workpiece surface, and is composed of a plurality of lenses 22. A laser beam 24 emitted from a laser oscillator 23 is linearly polarized light, and is changed to circularly polarized light while being changed in a direction thereof by the circularly polarizing mirror 20. The laser beam 24 changed to the circularly polarized light passes through the plurality of zero shift mirrors 21, and is guided to the processing optical system to be condensed at a desired position of a workpiece 25 by the lenses 22 of this processing optical system. In this case, even if ideal circularly polarized light can be produced by the circularly polarizing mirror 20, passing through the zero shift mirrors 21 and lenses 22 may change a polarization state little by little, which may impair the fine circularly polarized light. If the circularly polarized state has collapsed, decrease in machining accuracy such as decrease in roundness of a hole when the hole is formed, for example, is disadvantageously caused.
Even when a manufacturing error (especially, a phase shift) in optical characteristics of each of the mirrors satisfies a general optical specification, a final polarization state resulting from guiding the laser beam from the transmission optical system to the processing optical system may not attain an ideal circular polarization degree due to complex integration of phase shift errors in the respective mirrors. For example, as to phase shift specifications for the individual mirrors, if the phase shift is ±3° in the circularly polarizing mirror, and ±2° in the zero shift mirrors, and if respective required numbers of the elements are one and six, transmission mirror disposition that cannot reduce the error between the phase shifts of the mirrors causes a shift of at most 15° (3°×1+2°×6).
Against this, measures of making strict the phase shift specification of each of the mirrors have been conventionally taken so that a phase shift specification in the whole transmission optical system satisfies a predetermined standard, which results in a considerable number of defective products. Moreover, the transmission optical system is often customized for each laser beam machine, and thus, it is difficult to realize the phase shift specification applicable to all laser beam machines.
In this manner, there is a limit to making strict the phase shift specification of each of the mirrors, and it is difficult to say that making it strict is a realistic measure in view of a production error.
As a measure to increase machining capacity by eliminating directionality of machining by polarization, there has been proposed a technique of matching a direction of polarization of a laser beam to a direction of machining of a workpiece (refer to Patent Literature 1). In a laser beam machine described in Patent Literature 1, the laser beam is passed through two quarter waveplates, and the laser beam in a linearly polarized state is used for machining. One of the two waveplates is rotatable, and a method is employed in which the one waveplate is rotated so that the direction of the polarization is linked with a motion of the workpiece.
Similarly, as a technique of polarizing a laser beam to be used, which is different from laser machining, there has been an optical pickup apparatus. FIG. 5 is an explanatory view of an optical system of a general optical pickup apparatus. A laser beam 31 emitted from a semiconductor laser 30 is linearly polarized light, and is reflected at a polarization beam splitter 32 and is widened in a collimator 33. The laser beam 31 widened in the collimator 33 is converted to circularly polarized light in a quarter waveplate 34, and passes through an objective lens 35 to reach an optical disk 36. The laser beam 31, which has been subjected to intensity modulation and is reflected by irregularity of the optical disk 36, passes through the quarter waveplate 34 to be converted to linearly polarized light with a polarization plane perpendicular to an outward route. Thereby, the laser beam 31 is transmitted without being reflected at the polarization beam splitter 32, is condensed at a condensing lens 37, and reaches a photo-detector 38 to be converted to an electric signal in the photo-detector 38.
If the above-described optical pickup apparatus is operated for a long time, an oscillation wavelength of a laser oscillator may be shifted due to time degradatin. If the oscillation wavelength of the laser oscillator is shifted, there is a risk that a desired phase shift cannot be obtained at the quarter waveplate 34 inside the pickup, which leads to a decrease in optical energy and a reading error.
Consequently, there has been proposed a technique of using a phase delay plate with an organic thin film having birefringence (refer to Patent Literature 2). In an optical pickup apparatus described in Patent Literature 2, even if an oscillation wavelength of a laser oscillator is changed due to time degradation, the use of the organic thin film having birefringence raises a temperature of the organic thin film, following the change of the oscillation wavelength to thereby change a phase shift, which can suppress influence by time degradation.